I also remember in one of the mayday episodes, that on one aircraft the transponder control was somehow near the foot rest. So it was very common for pilots on this model aircraft to accidentally disable or change transponder codes when they put their feet on the foot rest.

Sorry I don't remember which aircraft model or episode this was.

Old cars used to have the high beam switch down there, a totally reasonable concept. A switch that instantly throws every pilot and controller in the area into panic mode? I want to meet the people who approved that and see what their Pro/Con list looked like.

I believe the transponder code can be changed to a special value to indicate a hijack condition. Perhaps they install a sort of "silent alarm" control to do this?

7500 - hence in light aircraft (with rotary dials to adjust each number) you have to switch the transponder to standby before changing squawk. Prevents briefly sending out a 75/76/7700 as you're rotating through them.